To meet the demand for increased operating efficiency, gas turbine engine designers would like to employ higher and higher operating temperatures. However, the ability to increase operating temperatures is often limited by material properties. One application with such a limitation is gas turbine transition ducts. Transition ducts are often welded components made of sheet or thin plate material and thus need to be weldable as well as wroughtable. Often gamma-prime strengthened alloys are used in transition ducts due to their high-strength at elevated temperatures. However, current commercially available wrought gamma-prime strengthened alloys either do not have the strength or stability to be used at the very high temperatures demanded by advanced gas turbine design concepts, or can present difficulties during fabrication. In particular, one such fabrication difficulty is the susceptibility of many wrought gamma-prime strengthened alloys to strain age cracking. The problem of strain age cracking will be described in more detail later in this document. Another potential fabrication difficulty is hot cracking, a problem which may occur during welding, particularly in alloys which contain a certain amount of zirconium.
Wrought gamma-prime strengthened alloys are often based on the nickel-chromium-cobalt system, although other base systems are also used. These alloys will typically have aluminum and titanium additions which are responsible for the formation of the gamma-prime phase, Ni3(Al,Ti). Other gamma-prime forming elements, such as niobium and/or tantalum, can also be employed. An age-hardening heat treatment is used to develop the gamma-prime phase into the alloy microstructure. This heat treatment is normally given to the alloy when it is in the annealed condition. The presence of gamma-prime phase leads to a considerable strengthening of the alloy over a broad temperature range. Other elemental additions may include molybdenum or tungsten for solid solution strengthening, carbon for carbide formation, and boron for improved high temperature ductility.
Strain age cracking is a problem which limits the weldability of many gamma-prime strengthened alloys. This phenomenon typically occurs when a welded part is subjected to a high temperature for the first time after the welding operation. Often this is during the post-weld annealing treatment given to most welded gamma-prime alloy fabrications. The cracking occurs as a result of the formation of the gamma-prime phase during the heat up to the annealing temperature. The formation of the strengthening gamma-prime phase in conjunction with the low ductility many of these alloys possess at intermediate temperatures, as well as the mechanical restraint typically imposed by the welding operation will often lead to cracking. The problem of strain age cracking can limit alloys to be used up to only a certain thickness since greater material thickness leads to greater mechanical restraint.
Several types of tests to evaluate the susceptibility of an alloy to strain age cracking have been developed. These include the circular patch test, the restrained plate test, and various dynamic thermal-mechanical tests. One test which can be used to evaluate the susceptibility of an alloy to strain age cracking is the controlled heating rate tensile (CHRT) test developed in the 1960's. Recent testing at Haynes International has found the CHRT test to successfully rank the susceptibility of several commercial alloys in an order consistent with field experience. In the CHRT test, a sheet tensile sample is heated from a low temperature up to the test temperature at a constant rate (a rate of 25° F. to 30° F. per minute was used in the tests run at Haynes International). Once reaching the test temperature the sample is pulled to fracture at a constant engineering strain rate. The test sample starts in the annealed (not age-hardened) condition, so that the gamma-prime phase is precipitating during the heat-up stage as would be the case in a welded component being subjected to a post-weld heat treatment. The percent elongation to fracture in the test sample is taken as a measure of susceptibility to strain age cracking (lower elongation values suggesting greater susceptibility to strain age cracking). The elongation in the CHRT is a function of test temperature and normally will exhibit a minimum at a particular temperature. The temperature at which this occurs is around 1500° F. for many wrought gamma-prime strengthened alloys.
Good strength and thermal stability at the high temperatures demanded by advanced gas turbine concepts are two properties lacking in many current commercially available wrought gamma-prime strengthened alloys. High temperature strength has long been evaluated with the use of creep-rupture tests, where samples are isothermally subjected to a constant load until the sample fractures. The time to fracture, or rupture life, is then used as a measure of the alloy strength at that temperature. Thermal stability is a measure of whether the alloy microstructure remains relatively unaffected during a thermal exposure. Many high-temperature alloys can form brittle intermetallic or carbide phases during thermal exposure. The presence of these phases can dramatically reduce the room-temperature ductility of the material. This loss of ductility can be effectively measured using a standard tensile test.
Many wrought gamma-prime strengthened alloys are available in sheet form today in today's marketplace. The Rene-41 or R-41 alloy (U.S. Pat. No. 2,945,758) was developed by General Electric in the 1950's for use in turbine engines. It has excellent creep strength, but is limited by poor thermal stability and resistance to strain age cracking. A similar General Electric alloy, M-252 alloy (U.S. Pat. No. 2,747,993), was also developed in the 1950's. Although currently available only in bar form, the composition would easily lend itself to sheet manufacture. The M-252 alloy has good creep strength and resistance to strain age cracking, but like R-41 alloy is limited by poor thermal stability. The Pratt & Whitney developed alloy known commercially as WASPALOY alloy (apparently having no U.S. patent coverage) is another gamma-prime strengthened alloy intended for use in turbine engines and available in sheet form. However, this alloy has marginal creep strength above 1500° F., marginal thermal stability, and has fairly poor resistance to strain age cracking. The alloy commercially known as 263 alloy (U.S. Pat. No. 3,222,165) was developed in the late 1950's and introduced in 1960 by Rolls-Royce Limited. This alloy has excellent thermal stability and resistance to strain age cracking, but has very poor creep strength at temperatures greater than 1500° F. The PK-33 alloy (U.S. Pat. No. 3,248,213) was developed by the International Nickel Company and introduced in 1961. This alloy has good thermal stability and creep strength, but is limited by a poor resistance to strain age cracking. As suggested by these examples, no currently commercially available alloys possess the unique combination of three key properties: good creep strength and good thermal stability in the 1600° F. to 1700° F. temperature range, as well as good resistance to strain age cracking.
British Patent Publication GB 1 029 609 discloses an alloy adapted for use in the manufacture of gas turbine engines. However, the claimed compositional range of the British patent includes compositions which would be expected to fail at least one of the three gas turbine transition key properties described above based on the relationships taught by the present invention. Furthermore, the publication does not teach how to control the composition to meet these desired properties. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Japanese Published Patent Application JP 01129942 discloses a nickel-based alloy said to have excellent hot workability. This publication teaches that zirconium improves the hot workability of the alloy and should be present in an amount of from 0.02% to 0.1%. But, such levels of zirconium are likely to produce hot cracking problems during welding of the alloy. This patent application also teaches that W is necessary for high temperature toughness, while I have found that W is not necessary for the key desired properties in a gas turbine transition duct, but may be present as a partial substitute for Mo according to a specified relationship. Moreover, the claimed compositional range of JP01129942 contains alloys which would be expected to fail one or more of the key properties for gas turbine transition ducts based on the relationships taught by the present invention. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Japanese Published Patent Application JP 06172900 discloses a nickel-chromium-cobalt-molybdenum alloy containing from 8% to 12% molybdenum. However, this patent (issued in the 1990's) appears to claim compositions of much earlier patented alloys such as R-41 and M-252 alloys (described previously in this publication). The reference fails to recognize that molybdenum levels above 9.1% can lead to lower thermal stability and lower creep strength in this type of alloy. The claimed compositional range of this Japanese reference includes compositions which I have shown do not meet at least one of the key properties described above. Furthermore, the publication does not teach how to control the composition to meet these desired properties. Finally, no example alloys from that publication fall within the preferred ranges of the present invention.
Consequently, there is a need for an alloy which has a combination of good creep strength, good stability in the 1600° F. to 1700° F. temperature range, good resistance to strain age cracking and which can be welded without encountering hot cracking problems.